In amplifiers for ultrashort optical pulses, the occurring optical peak intensities can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or of some other optical element may occur. This can be effectively prevented by employing the method of chirped-pulse amplification.
Before passing the amplifier medium, the pulses are chirped and temporally stretched to a much longer duration by means of a strongly dispersive element (the stretcher, e.g. a grating pair or a long fiber). This reduces the peak power to a level where the above mentioned detrimental effects in the gain medium are avoided. After the gain medium, a dispersive compressor is used, i.e., an element with opposite dispersion (typically a grating pair), which removes the chirp and temporally compresses the pulses to a duration similar to the input pulse duration. As the peak power becomes very high at the compressor, the beam diameter on the compressor grating has to be rather large. For the most powerful devices, a beam diameter of the order of one meter is required.
The method of chirped pulse amplification has allowed the construction of table-top amplifiers which can generate pulses with millijoule energies and femtosecond durations, leading to peak powers of several terawatts. For the highest peak powers in ultrashort pulses, amplifier systems consisting of several regenerative and/or multipass amplifier stages are used, which are mostly based on titanium-sapphire crystals. Such amplifiers can be used e.g. for high harmonic generation in gas jets. Large-scale facilities even reach peak powers in the petawatt range.
When ordinary holographic diffraction gratings are used for the compressor, the four reflections on gratings can easily cause a loss of approximately 50%. In order not to lose half of the power at the end, special transmission gratings, fabricated with electron beam lithography, have been developed with losses of only approximately 3% or even less per reflection (at least for one polarization direction), resulting in much better efficiency of chirped-pulse amplifier systems. Another possibility is to use volume Bragg gratings. A single such grating can be used as the stretcher and compressor.
Another approach to reduce the compressor losses is down chirped pulse amplification, where the stretcher uses anomalous dispersion so that the compressor can be a simple glass block with normal dispersion.
For ultrabroad optical spectra, as are associated with few-cycle laser pulses, the main challenge of the CPA technique is to obtain a sufficiently precise match of the dispersion of stretcher and compressor despite the large stretching/compressing ratio. This is difficult due to higher-order chromatic dispersion. On the other hand, systems for relatively long (picosecond) pulses require enormous amounts of chromatic dispersion, which are not easily provided. Therefore, CPA systems work best for pulse durations between roughly 20 fs and a few hundred femtoseconds.
The concept of chirped pulse amplification is also applied to fiber amplifiers. Due to the inherently high nonlinearity of long fibers, CPA has to be applied already for relatively low pulse energies, and even with strong temporal stretching of the pulses, the achievable pulse energies stay quite limited. However, high average powers of tens of watts or even >100 W can be generated. Fiber-based CPA systems are therefore most suitable for high pulse repetition rates combined with high average powers. The fibers used for such systems should be optimized in various respects; they should have features such as e.g. a high gain per unit length, polarization-maintaining properties (strong birefringence), core-less end caps, etc.
Unfortunately, all-fiber solutions are normally not possible, since the temporal compression has to be done with a dispersive compressor with a mode area well above that of a fiber. There is some progress, though, towards air-guiding photonic crystal fiber compressors, which at least allow significantly higher pulse energies than previously considered to be realistic for fibers.